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Single-Molecule Detection of a Cyanine Dye in Silver Colloidal Solution Using Near-Infrared Surface-Enhanced Raman Scattering

Volume 52, Number 2 (Feb. 1998) Page 175-178

Kneipp, Katrin; Kneipp, Harald; Deinum, Geurt; Itzkan, Irving; Dasari, Ramachandra R.; Feld, Michael S.

Single-molecule Raman spectroscopy of a cyanine dye in aqueous silver colloidal solution with the use of surface-enhanced Raman scattering at near-infrared excitation (NIR-SERS) is reported. A characteristic Poisson distribution of SERS signals due to the Brownian motion of single dye molecule-loaded silver particles reflects the probability of finding 0, 1, or 2 1,1 ' -diethyl-2,2 ' cyanine (PIC) molecules in the probed volume during an actual measurement and is evidence that single-molecule detection by SERS has been achieved. Spectra measured in 1 s collection time with 100 mW nonresonant 830 nm excitation provide a clear ''fingerprint'' of a single PIC molecule by showing its typical Raman lines between 700 and 1700 cm -1. Single-molecule Raman signals are also detected for the first time at the anti-Stokes side of the excitation laser. Effective Raman cross sections for PIC of 10 -16 cm2 per molecule can be inferred from the ratio between ''pumped'' anti-Stokes and Stokes signals.